1. Field of the Invention
The invention concerns a process for equalizing the distortion of a received signal.
2. Description of the Art
In transmission channels with multi-path propagation, the receiver must equalize the received signal prior to the evaluation of the information content. Various versions of equalizer filters are known; see for example the overview in Digital Medium Wave Radio Broadcasting from A. Brakemeier in telecom praxis 9/96, pages 33-38. For approximation of the channel impulse response (channel unit pulse response), which is necessary for setting the equalization filter, test sequences of known structure are incorporated in the transmitted signal, in particular so called PN-sequences or CAZAC-sequences.
One frequently employed equalization process is equalization with decision feedback (Decision Feedback Equalizer). The known equalization processes work satisfactorily in the case of transmission channels with trailing or lag signals which are weaker than the main signal, however show significant problems in particular in the case of the reverse signal power distribution.
The present invention is thus concerned with the task of improving a process of the above-described type in a manner that reduces the dependency of the equalization effectiveness on the structure of the channel impulse response.
The invention provides a process for equalization of a signal received via a time varying transmission channel and scanned via an equalizer filter (DFE), of which the filter coefficients are tuned or adjusted according to the value of an approximation of the channel impulse response on the basis of test sequences (TF) embedded in the signal which form signal frames (Rn, Rn+1) with data sequences (DF) enclosed between them, and thereby characterized, that from the approximation of the channel impulse response a determination is made as to the utilization of one of at least two different process paths for equalization, wherein it is possible to select between at least one forward equalization and one inverse-time reversed equalization.
The invention makes it possible, with small expenditure, to employ a particularly suitable alternative from among the many mentioned above, for carrying out equalization. The component and energy cost can be kept particularly small by allowing for only two different alternativesxe2x80x94in particularxe2x80x94one forwards equalization according to the conventional manner of proceeding, and one inverse-time reversed equalization. The decision between the different alternatives can be considered as a classification problem, for which a classifier can be set up. An advantageous and economical criteria for classification is the so-called slope or obliquity of the channel impulse response, or a function derived therefrom, which is simple to calculate and of which the plus or minus value provides, in a preferred embodiment of the invention, the selection of one of the two alternatives for equalization.
The allowance of a second, inverse-time reversed equalization besides the conventional forwards equalization has the advantage that for calculation of the filter coefficients for both alternatives the same rules for calculation can be utilized, with the exception of the supplemental or additional time inversion. The time loss due to the intermediate storage or buffering of the sample values or the therefrom derived values of a frame can be disregarded, since in any case as a result of the conventional interleaving and scrambling of the symbols an evaluation lag occurs. The frame-wise processing during the equalization offers beyond this still further advantages, for example, the taking into consideration of two channel impulse responses obtained using different test procedures.